Owing to the type of missions carried out, landing and takeoff in zones that are difficult to access, sometimes unprepared, or flight at low altitude, a helicopter, for example, is a craft which is very highly exposed to the risk of collision with obstacles situated in its close environment. Beyond the geographical aspect, during medical evacuation operations, the use of the helicopter is quite often reserved for emergency survival cases for which swiftness of action and the continuation of the mission are vital in respect of the victim to be rescued. The urgent nature of the mission and the taking of risks which stems therefrom, correspondingly increase the risks of being in proximity to obstacles.
The person skilled in the art is familiar with systems of TAWS type, “Terrain Awareness and Warning System”. The aim of these systems is to generate an alert when the aircraft is in a dangerous situation where the operational margins are no longer complied with TAWSs in the guise of autonomous computer or computer integrated with the TCAS (Traffic Collision Avoidance System) function and WXR (Weather Band X Radar) function, in an ISS (Integrated Surveillance System), fulfill a primary function of terrain anti-collision surveillance (“Safety Net”) and the aim of which is the emission of audible alerts upon an exceptional approach to the relief allowing the crew to react by engaging a vertical resource before it is too late.
Accordingly, the TAWS systems, decoupled from navigation systems, proceed in two ways. They periodically compare the theoretical trajectory that would be described by the aircraft during a resource and compare it with a sectional cut through the terrain and with the obstacles overflown obtained on the basis of a worldwide digital terrain model embedded aboard the computer. Or then, some TAWSs also integrate modes termed “reactive modes” which, by periodically comparing some of the current parameters of the craft, for example the radio-altitude and the vertical speed, various charts determine whether the current situation of the aircraft is a normal situation or whether it is potentially dangerous. In the latter case, an alert, limited to a verbal message, is generated to inform the crew. The availability of a model of the terrain permits functions making it possible to improve the perception of the situation of the crew. Among them, the alert lines have the objective of delimiting the terrain zones for which a TAWS alert might appear. For their part, the “Alert Areas” show the zones giving rise to a TAWS alert. These alert lines are displayed in the navigation display device, commonly called the “navigation display”.
Beyond the logic of alerts, the requirements expressed by pilots are to have at their disposal graphical information allowing them to remain outside of situations that may evolve dangerously.
One known at-risk obstacle zone detection system compares the position of an obstacle in relation to the current position of the aircraft. The calculation method for determining the risk of the dangerous zones does not take into account the current behavior of the aircraft, nor the capabilities of the aircraft. Moreover, the display of the zones of obstacles shows the zones for which alerts are already engaged.
A display of the zones at risk for which alerts are already engaged aboard the aircraft is also known.